pub fn example() {
    let mut _mutable_integer = 7i32;
    {
        //被不可变的遮蔽之后
        let _mutable_integer = _mutable_integer;
        //本作用域被冻结
        //_mutable_integer = 50;
    }
    //只是冻结了上述代码块中的作用域
    _mutable_integer = 3;
}